1. Field of the Invention
The invention relates to drug delivery implantable medical devices, one example of which is a stent. More particularly, the invention relates to selectively coating a device to accommodate differences in strain experienced by different portions of a device during use.
2. Description of the Background
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to remodel the vessel wall. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient's vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings, which can collapse and occlude the conduit after the balloon is deflated. Vasospasms and recoil of the vessel wall also threaten vessel closure. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may necessitate another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis a stent is implanted in the lumen to maintain the vascular patency.
Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically, stents are capable of being compressed or crimped onto a catheter so that they can be delivered to and deployed at a treatment site. Delivery includes insertion through small lumens via a catheter and transporting the stent to the treatment site. Deployment includes expanding the stent to a larger diameter once it is at the desired location. Mechanical intervention via stents has reduced the rate of restenosis as compared to balloon angioplasty. Yet, restenosis remains a significant problem. When restenosis does occur in the stented segment, its treatment can be challenging, as clinical options are more limited as compared to lesions that were treated solely with a balloon.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or even toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results.
To fabricate a conventional coating, a polymer, or a blend of polymers, can be applied on the stent using techniques known to those having ordinary skill in the art. A composition for applying to a stent may include a solvent, a polymer dissolved in the solvent, and an active agent dispersed in the blend. The composition may be applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the active agent impregnated in the polymer. Selective coating of an implantable medical device, such as a stent, is described herein.
A potential shortcoming of the foregoing method of medicating stents is that a polymeric drug coating disposed directly on the surface of the stent may not attach well to the surface during crimping, deployment, or implantation of the stent as well as while the stent is in a patient. Some polymers applied on stents, for example, are relatively brittle at biological conditions (i.e., are capable of relatively small percent elongation before fracture). During deployment, the polymeric stent coating can be exposed to stress caused by the radial expansion of the stent body. In addition, a polymeric stent coating may be exposed to stress when it is mounted on a catheter from crimping or compression of the stent. These stresses can cause the coating to tear or fracture. Failure of the mechanical integrity of the stent while the stent is localized in a patient can lead to a serious risk of embolization caused by a piece of the polymeric coating breaking off from the stent. Polymeric stent coatings having a high drug loading are especially vulnerable to fracture during and after deployment. Active agents tend to increase the crystallinity of a coating. As a result, elasticity of the coating may be decreased which makes the coating more susceptible to failure when subjected to high stress.
It is therefore desirable to improve the adhesion or retention of the polymeric coating to the surface of a stent. It is also desirable to be able to increase the quantity of the therapeutic substance carried by the polymeric layer without perturbing the mechanical properties of the coating, such as inadequate coating adhesion. It is additionally desirable to provide an improved polymeric coating that is capable of delivery and expansion with a stent without any or significant detachment from the surface of the stent. The present invention meets the foregoing as well as other needs.